A TFT is a field effect semiconductor device, and comprises important portions like a substrate, a gate, a gate insulating layer, an active layer, a source, and a drain. The active layer plays a key role in the device performance and the fabricating process. In a recent decade, a liquid crystal display device in which a Si-TFT is used as the driving unit has developed rapidly and become a popular information display terminal, due to its advantages of small volume, light weight, and high quality. However, amorphous silicon has the disadvantages of low field-effect mobility, sensitive to light, and opacity of material, while the process for fabricating a polycrystalline silicon TFT with a large area is complex and a low temperature process is difficult to realize.
However, in the current TFT, an oxide semiconductor in the active layer degrades if exposed to light. Under irradiation by a backlight source, light passes through a substrate impinges on a source and drain metal layer, and then reflects to impinge on a gate metal layer. This continuous reflection causes light to impinge on a semiconductor layer. Since the semiconductor layer is extremely sensitive to light, the characteristics of the TFT will degrade significantly after irradiation for a long time.